The present invention relates generally to methods and apparatus for extruding thermoplastic material and, more particularly, to methods and apparatus for monitoring and controlling deviations in the output of a thermoplastic extruder from predetermined desired extruder output parameters.
Many various forms of thermoplastic polymeric material are conventionally extruded into a wide variety of products including sheets, film, rods, tubes, strands as well as various other products. Basically, the extrusion process involves melting of the thermoplastic material employed, and sufficient pressurization of the melted material to cause it to flow, at a desired rate, through an appropriate die to form the intended end product. An important determination of the quality of the end product is its conformity, within acceptable tolerances, to predetermined dimensional and weight specifications. As a general principle, the quality of an extruded product tends to decrease as the extrusion rate increases. Over the years, it has therefore been an ongoing problem in the industry in striking an acceptable balance between an economical rate of extrusion and an acceptable quality of the extruded product.
Conventional apparatus for extruding thermoplastic material typically includes a cylindrical barrel in which is rotatably disposed a conveying screw. The thermoplastic material is fed is a solid pellet or powder form into the barrel at one end and conveyed to the other end of the barrel by rotation of the screw. The heat required to transform the thermoplastic material from a solid at room temperature to a molten material at the desired extrudate temperature is typically generated by equipping the extruder with heating elements.
Conventional extruders are widely recognized to be disadvantageously subject to undesirable deviations in the quality of the extruded product. While the volumetric input of thermoplastic material to the extruder and the operational parameters of the extruder, e.g., rotational speed of the screw and temperature of the extruder heaters, may be maintained substantially constant, variations in the thermoplastic material such as its density and viscosity as well as variations in the operating conditions of the extruder such as uneven temperature distribution and backpressurization cause unacceptable variations in the quality of the extruded product exceeding established tolerances. When such variations result in an extruded product exceeding established thickness specifications by more than the acceptable tolerance, the extruder utilizes an excessive amount of thermoplastic material and, in turn, the profitability of the extruder suffers. When such variations result in an extruded product of a thickness less than established thickness specifications by more than the acceptable tolerance, the product is defective and unsalable.
In recent years, various techniques and approaches have been developed to attempt to control deviations in the output of extrusion systems to avoid these problems. One such approach is to utilize a gear pump following the extruder to meter the delivery of the molten thermoplastic material to the forming die. The basic construction and operation of gear pumps is well known, basically including a pair of counter-rotating intermeshing gears driven within a pump housing to transport the molten thermoplastic material in a positive displacement manner by entrapping the material between the teeth of the gears. As a result, the volumetric output of a gear pump is a function of its operational speed and is substantially unaffected by pressure surges and fluctuations in the input flow of thermoplastic material from the extruder. As a result, the use of a gear pump reduces variations in the quality of the extruded product, both on short term and long term bases, by delivering a substantially uniform volumetric flow of thermoplastic material to the die of the extrusion system. However, variations in the density of the thermoplastic material are substantially unaffected by the use of a gear pump and, therefore, such variations may still product deviations in the weight per unit length of the extruded product.
More recently, considerable interest has developed in the utilization of so-called gravimetric extrusion control systems as an alternative to gear pump-assisted extrusion control. Basically, gravimetric extrusion control utilizes a special hopper for weighing the material input into the extruder over the course of extruder operation to determine the rate of material consumption by the extruder, with variations in the extruder consumption being utilized to adjust the rotational speed of the extruder screw to more uniformly control the rate at which the extruder output of molten thermoplastic material is delivered to the die. Disadvantageously, however, gravimetric control systems are generally ineffective for controlling short term fluctuations in the product output of an extruding system.